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APPENDIX                                     273

upon ethyl bromide. The presence of 7W//-0- groups, however,
disturbs this stability, and the halogen in a substance like
dinitrochloroben/ene is readily replaced by hydroxyl with
potash, or by N II., with ammonia. When the halogen is in the
side-chain, the substance behaves like an aliphatic compound.

Ethyl Benzene.--" Fittig's reaction," so-called from its
discoverer, is analogous to the synthetical method employed by
Wurt/. for the preparation of the aliphatic hydrocarbons, as in
the formation of butane from ethyl bromide^
2( U Ir,I5r -I- 2Nn = Cy 1 10 -I- 2NaBr.
In the case of the aromatic hydrocarbons, a second side-
chain may be introduced from ,-r dibrornodefivative cither
simultaneously with the first, or subsequently by a repetition of
the process. Both dibromobenzcne and monobromotolucnc
may be converted into xylene.
t.y I4I{r.j -I- 2(:n,I   ! 4N;i .-..- QlI^CIIj^ + 2Nar H- 2'NaI.
Crtn.iHr(;H:,   -I-   CH.J   I  2Na :.-- (Y,II4(CII,,),s -I- Nul -I- Na'Bn
The action  also takes  place between aromatic  hydrocarbons
substituted either in the nucleus or side-chain.     Urumobenzcne
yields diphenyl, wliereas benxyl bromide yields dibenzyl,
2Cfi!I6I{r I- 2Na .- CflIIft.CVtB -I- 2NaIk.
2Q;I I5CI I,Hr f 2Na ^ C(i! Ifl.CIta.Clla.C,jir5 ~f aNaBr.
This reaction does not, however, occur with the same readiness
in all cases, nor does it always yield exclusively the anticipated
product. Para-broinotoluene and sodium give tolyl phenyl
methane and diben/.yl as well as ditolyl (Wciler). Again, p-
broniololuene gives a good yield of /-xylenc, the ortho-compound
reacts sluggishly, whilst the meta-derivative gives no xylene.
O<x:asionally the action is vigorous, and has to be moderated by
dilution with an indifferent solvent. At other times it is sluggish
and has to be promoted by raising' the temperature. Often the
addition of a little ethyl acetate will start the decomposition.
For the synthesis of ;;omc of the aromatic hydrocarbons, it is
preferable to 'use the Friedel-Crafts' reaction (see Prep. 102,
]> 214).
COHEN'S ADV. p. o. c.                                                  T